Slip Element and Assembly for Oilfield Tubular Plug

ABSTRACT

A slip assembly for anchoring a sealing device, such as a bridge plug, frac plug, or packer, in an oilfield tubular. The slip assembly includes a polymeric slip cone and slip stop disposed along a mandrel. Plural wedge shaped slip elements, which each have plural teeth for selectively engaging the interior circumference of the tubular, are pressure molded of powered iron that is sintered. The plural slip elements are spaced apart from each other such that the sum length of the spaces between them along the interior circumference of the tubular accounts for greater than thirty percent of the circumference. The teeth of the plural slip elements are driven against the interior circumference of the tubular when the slip cone and the slip stop are driven toward one another along the mandrel by a setting tool.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to plug tools for petroleum well tubulars.More particularly, the present invention relates to pressure sealingbridge plugs, ‘frac’ plugs and packers that employ all polymericcomponents, except for the use of sintered metal slips that are spacedapart to facilitate setting and removal by drilling after deployment.

Description of the Related Art

Oil and gas well construction begins with a wellbore drilled into theground to a predetermined depth. The wellbore is lined with a steel wellcasing, which is commonly cemented in place within the wellbore. Beforethe well is placed into production, the casing is perforated at one ormore depths to enable well fluids to flow from the formation into thewell casing. Various tools may be run down the casing to develop thewell and to commence production of hydrocarbon minerals, and to maintainthe well over the years. Depending on the petroleum fluid bearingformation into which the well is drilled, various sequences of tools maybe used. For example, in the case of well that has well fluids dispersedinto a porous formation, hydraulic fracturing may be employed tofacilitate the migration of well fluids into the well casing.

A common requirement in well development and servicing is the need toseal the casing, or other petroleum tubular, against pressure to controlmovement of fluids between two sides of a particular location in thetubular. For example, in the case of hydraulic fracturing operations, itis necessary to plug the casing below the perforations into whichfracturing fluid is pumped in order to assert the requisite pressureneeded to fracture the formation. This type of tool is referred to as a“frac” plug. More generically, the term “bridge plug” may be used.Another pertinent term of art is the “packer”, which is a specializedplug used in an annulus between tubulars. Generally, these may simply bereferred to as “plugs”. Hydraulic fracturing is just one example, andthose skilled in the art are familiar with numerous applications for theuse of pressure sealing plugs in oil and gas well operations.

A plug is typically lowered into the tubular using a line, such as awire line, and is then set into place using a setting tool. There arevarious setting tools available, but a common mode of operation is asetting tool that engages the plug, and then applies downward force fromabove the plug while pulling upwardly on a central mandrel of the plug.This action generates high compressive forces within the plug, and thisforce is used to compress a sealing member of the plug against theinterior of the tubular. Such plugs generally incorporate plural slipsthat bite into the interior circumference of the tubular as the plug isset, and serve to hold the plug in position and also to hold compressiveforces on the sealing element to perfect the seal while the pressureoperations are undertaken.

Once the pressure operation is completed, the plug generally must beremoved so that subsequent operations can be undertaken. A common methodto accomplish this is to drill the plug out of the tubular using a welldrilling bit such as the common tricone drill bit. The drill bit islowered to the plug location and then run to grind the plug into smallpieces, which may either be pumped to the surface using a liquid, orallowed to fall to the bottom of the wellbore. This brings into issuethe choice of plug materials, as well as the physical configuration ofthe plug. There is also a significant cost factor because plugs arewidely employed and are generally single use tools. Simple metal plugsare low cost, but take longer to drill out and are harder on the drillbits. Drill bits are not indestructible and gradually wear, so it ispreferable to manufacture plugs out of materials that can be easilydrilled, that can be drilled quickly, and that fragment into relativelysmall pieces. Polymeric materials have been employed, but presentchallenges with respect to strength and cost factors. Particularly withrespect to the requirement that the slips bite into the steel tubularmaterial sufficiently to hold the plug and resist high differentialpressures.

Thus it can be appreciated that there is a need in the art for apetroleum tubular pressure plug that enables the pressure sealingrequirements, and is readily removable by drilling.

SUMMARY OF THE INVENTION

The need in the art is addressed by the apparatus of the presentinvention. The present disclosure teaches a slip assembly for anchoringa sealing device that is assembled about a mandrel and set in place in atubular with a setting tool, where the tubular has an interiorcircumference. The slip assembly includes a slip cone fabricated from apolymeric material this is disposed along the mandrel, and a slip stopfabricated from a polymeric material also disposed along the mandrel.The assembly also includes plural wedge shaped slip elements that eachhave plural teeth disposed on an arcuate surface for selectivelyengaging the interior circumference of the tubular, which are pressuremolded of powered iron that is sintered. The plural slip elements aredisposed between the slip cone and the slip stop, and are spaced apartfrom each other. The teeth of the plural slip elements are drivenagainst the interior circumference of the tubular when the slip cone andthe slip stop are driven toward one another along the mandrel by thesetting tool. And wherein, the sum length of the spaces between theplural slip elements along the interior circumference of the tubularaccounts for greater than thirty percent of the interior circumferencelength of the tubular.

In a specific embodiment of the foregoing apparatus, the powered ironfurther contains carbon, and the arcuate surface of the plural slipelements is surface hardened. In a refinement to this embodiment, theplural slip elements are molded from powdered iron comprising carbon inthe range of 0.6 to 0.9 percent by weight, and also comprising poweredcopper in the range from 1.5 to 3.9 percent by weight, and, the pressuremolded plural slip elements are heat sintered to a temperature above themelting point of copper. In another refinement, the arcuate surface ofthe plural slip elements are surface hardened by oxy-gas flame andrapidly cooled to yield a case surface with a Rockwell C-scale hardnessvalue that is greater than fifty-five.

In a specific embodiment of the foregoing apparatus, the sum length ofthe spaces between the plural slip elements along the interiorcircumference of the tubular accounts for greater than fifty percent ofthe interior circumference length of the tubular.

In a specific embodiment, the foregoing apparatus further includesplural pins disposed between the plural slip elements and the slip coneto rigidly fix the plural slip elements to the slip cone, and the pluralpins have a shear strength selected to shear under force of the settingtool. In another specific embodiment, the foregoing apparatus furtherincludes plural pins disposed between the plural slip elements and theslip stop to rigidly fix the plural slip elements to the slip stop, andthe plural pins have a shear strength selected to shear under force ofthe setting tool.

In a specific embodiment, the foregoing apparatus further includesplural pins disposed between the plural slip elements and the slip stopand the slip cone to rigidly fix the plural slip elements to the slipstop and the slip cone, and the plural pins have a shear strengthselected to shear under force of the setting tool. In a refinement tothis embodiment, the plural pins are fabricated from fiber reinforcedpolymeric material.

In a specific embodiment of the foregoing apparatus, the slip cone andslip stop polymeric materials are fiber reinforced, and the mandrel isfabricated from a fiber reinforced polymeric material.

The present disclosure teaches a slip assembly for anchoring a sealingdevice that is assembled about a fiber reinforced polymeric mandrel andset in place in a tubular with a setting tool, the tubular having aninterior circumference. The slip assembly includes a slip conefabricated from a fiber reinforced polymeric material, and a slip stopfabricated from a fiber reinforced polymeric material, both disposedalong the mandrel. The assembly also includes plural slip elements, eachconfigured in a wedged shape, that each have plural teeth disposed on anarcuate surface thereof for selectively engaging the interiorcircumference of the tubular. The plural slip elements are pressuremolded from powdered iron including carbon in the range of 0.6 to 0.9percent by weight, and also including powered copper in the range from1.5 to 3.9 percent by weight. The pressure molded plural slip elementsare heat sintered to a temperature above the melting point of copper,and the arcuate surface of the plural slip elements are surface hardenedby oxy-gas flame and rapidly cooled to yield a case hardened surfacehaving a Rockwell C-scale hardness value greater than fifty-five. Theassembly further includes plural pins, fabricated from a fiber reinforcepolymeric material, disposed between the plural slip elements and theslip stop and the slip cone to rigidly fix the plural slip elements tothe slip stop and the slip cone. The plural pins have a shear strengthselected to shear under force of the setting tool. The plural slipelements are spaced apart from each other such that the sum length ofthe spaces between the plural slip elements along the interiorcircumference of the tubular accounts for greater than thirty percent ofthe interior circumference length of the tubular. In operation, theteeth of the plural slip elements are driven against the interiorcircumference of the tubular when the slip cone and the slip stop aredriven toward one another along the mandrel by the setting tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view drawing of a bridge plug according to anillustrative embodiment of the present invention.

FIG. 2 is a partial exploded view of a bridge plug according to anillustrative 15 of the present invention.

FIGS. 3A, 3B, 3C, and 3D are a front view, side view, top view andbottom view drawing, respectively, of a bridge plug slip according to anillustrative embodiment of the present invention.

FIGS. 4A and 4B are a top view and section view drawing, respectively,of a bridge plug slip cone according to an illustrative embodiment ofthe present invention.

FIGS. 5A and 5B are a top view and section view drawing, respectively,of a bridge plug slip stop according to an illustrative embodiment ofthe present invention.

FIG. 6 is a side section view drawing of a slip stop assembly in a wellcasing according to an illustrative embodiment of the present invention.

FIG. 7 is a top section view drawing of a slip stop assembly in a wellcasing according to an illustrative embodiment of the present invention.

FIG. 8 is a side section view drawing of a deployed slip stop assemblyin a well casing according to an illustrative embodiment of the presentinvention.

FIG. 9 is a top section view drawing of a slip stop assembly in a wellcasing according to an illustrative embodiment of the present invention.

FIG. 10 is a top section view drawing of a deployed slip stop assemblyin a well casing according to an illustrative embodiment of the presentinvention.

FIG. 11 is a top section view drawing of a deployed slip stop assemblyin a well casing according to an illustrative embodiment of the presentinvention.

FIG. 12 is a top section view drawing of a deployed slip stop assemblyin a well casing according to an illustrative embodiment of the presentinvention.

DESCRIPTION OF THE INVENTION

Illustrative embodiments and exemplary applications will now bedescribed with reference to the accompanying drawings to disclose theadvantageous teachings of the present invention.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, and embodimentswithin the scope hereof and additional fields in which the presentinvention would be of significant utility.

In considering the detailed embodiments of the present invention, itwill be observed that the present invention resides primarily incombinations of steps to accomplish various methods or components toform various apparatus and systems. Accordingly, the apparatus andsystem components and method steps have been represented whereappropriate by conventional symbols in the drawings, showing only thosespecific details that are pertinent to understanding the presentinvention so as not to obscure the disclosure with details that will bereadily apparent to those of ordinary skill in the art having thebenefit of the disclosures contained herein.

In this disclosure, relational terms such as first and second, top andbottom, upper and lower, and the like may be used solely to distinguishone entity or action from another entity or action without necessarilyrequiring or implying any actual such relationship or order between suchentities or actions. The terms “comprises,” “comprising,” or any othervariation thereof, are intended to cover a non-exclusive inclusion, suchthat a process, method, article, or apparatus that comprises a list ofelements does not include only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. An element proceeded by “comprises a” does not,without more constraints, preclude the existence of additional identicalelements in the process, method, article, or apparatus that comprisesthe element.

The present disclosure teaches an improved slip and slip assemblysuitable for use in sealing plugs utilizes in petroleum well drilling,development, and production operations. Such plugs include, but are notlimited to, bridge plugs, ‘frac’ plugs, and packers. A common mode ofoperation is to run the plug down hole, such as into a steel wellcasing, with a setting tool attached. When a predetermined depth isreached, the plug is set by the setting tool to provide a pressure sealat the predetermined depth. After the pressure operation, such aspumping high-pressure hydraulic fracture fluid into a geologicformation, is completed, the plug needs to be removed to clear the wayfor subsequent down-hole operations. Most plugs are removed from thewell casing, (generically a “tubular”) by drilling.

Well drilling is accomplished with a drill bit. While well drill bitsare manufactured from durable components that are capable of cuttingrather hard materials, the drill bits do wear out. Drill bits arerelatively expensive. There is also a time factor involved with respectto how long it takes to drill out a plug. As would be expected,operators prefer a plug that is readily drilled out with minimal wearand tear on the drill bit, and in minimal operating time. It is furthernoteworthy that since the plug is a one-time use, disposable item, lowcost is a paramount requirement in the plug design. The materials thatare used to build the plug directly affect all of these foregoingfactors. Iron and steel are strong and inexpensive, but are difficult todrill out. Composite materials range higher in cost, and presentstrength limitations in the plug design.

A sealing plug comprises several components to achieve its intendedfunction. An elastomeric sealing member is provided, which is compressedagainst the interior circumference of the tubular to achieve thepressure sealing function. In lay terms, the sealing member is a‘rubber’ donut disposed on a mandrel. The sealing member is held alongthe mandrel between a pair of sealing shoes that are shaped to engageand compress the sealing member in an efficient manner. The sealingmember is squeezed between the sealing shoes. In addition to the sealingfunction of the plug, the plug also requires a holding function thatfixedly locates the plug during the pressure operation. The forcesagainst the holding function are high, and can reach tens of thousandsof pounds. The holding function is achieved using a pair of slipassemblies, one above and one below the sealing member. As the sealingmember is compressed, the slips are also driven against the interiorcircumference of the tubular, so as to ‘bite’ into the steel and holdthe plug in place, even after the setting tool is removed.

The slip assemblies in a plug engage the interior circumference of thetubular is a ratchet-like fashion, meaning that they are directional.The upper slip bites into the tubular at an angle that resists upwardmovement, and the lower slip bites into the tubular at an angle thatresists downward movement. This arrangement facilitates the functions ofholding the sealing member in compression, thereby sealably engaging thetubular. The slips are driven into the tubular using an inclined planearrangement. The slips are generally wedged shape and engage a slipcone, and together, these form the inclined plane. A slip stop islocated against each slip assembly opposite of the sealing member, andserve as the anchor points from the setting operation. The bottom slipstop is fixed to the mandrel, and the upper slip stop is moved towardthe lower slip stop by the setting tool. Essentially, the setting toolpulls upwardly in the mandrel while pushing downwardly on the upper slipstop, thereby squeezing the plug assembly in compression.

In an illustrative embodiment of this disclosure, the various componentsdisposed along the mandrel, as well as the mandrel itself, arefabricated from polymeric materials, some or which are fiber reinforced.This includes the mandrel, seal shoes, slip cones, slip stops, spacersand certain other components. For example, epoxy or phenolic resins canbe used. And, there is a range of other organic plastics that aresuitable. Reinforcing fibers may be glass, carbon, aramid, or othersuitable non-metallic fibers. All of these materials are relatively easyto drill out after the plug is deployed. However, in the case of theslips, the use of purely polymeric material is challenging because thehardness and compressive strength is generally insufficient toaccommodate the compressive forces at the time the plug is set andbiting forces into the steel tubulars. There have been a number of priorart attempts to incorporate hardened teeth into polymeric slips, andthese are disclosed together with the filing of this disclosure. While ahybrid approach may function adequately, they drive up cost and makesuch plugs less competitive in the marketplace.

The present disclosure presents new slip designs, as well as slipassembly designs, that effectively trade between strength, cost, anddrillability. A slip assembly is generally the combination of pluralslips, a slip cone, and a slip stop, which together enable the requisitesetting, holding, and drilling aspects of pressure plug utilization.Generally, a single plug as two slip assemblies disposed along itsmandrel, although other numbers of slip assemblies can certainly beemployed. Powdered metal slip components are employed which present asuitably hard biting surface to engage the interior of the tubular, butwhich are more easily drilled out due in part to the porous nature ofsintered metals. Surface hardening of the biting surface may be employedto ensure the slips can adequately grip the interior of the tubular.Sintered iron slips are particularly cost effective.

In addition to the use of sintered metal slips, the present disclosurealso teaches that the spacing between slips may be configured tofacilitate drilling after the plug has been deployed. In the prior art,plural slips are assembled together in the fashion of slices of pie, onesquarely against the next. If such slips are engaged by a drill bit,such as the familiar tricone drill bit, the slips tend to support oneanother, forcing the drill to grind through them all, taking time andwearing the drill bit. It should be noted that generally, an operatordoes not distinguish between bits of debris from a drilled out plug thatare pumped upwardly in the drilling liquid or that falls downwardly tothe bottom of the well casing. It is more important that the plug isremoved quickly and efficiently so that subsequent well operations maybeundertaken. For this to occur, the drill bit needs to effectively grindthe plug into suitable small pieces that either get pumped up, or falldown. By providing an adequate spacing between the slips, the drill bitcan shift and rotate the slips to more quickly facilitate the crumblingof the plug assembly into debris of suitable size.

Reference is directed to FIG. 1, which is an exploded view drawing of abridge plug 2 according to an illustrative embodiment of the presentinvention. Unless otherwise stated, all of the components in FIG. 1 arefabricated from polymeric materials, which may be fiber reinforced. Asmentioned hereinbefore, these include epoxy resins, phenolic resins, andother suitable organic plastics. Reinforcing fibers may be glass,carbon, aramid, or other suitable non-metallic fibers. A cylindricalmandrel 4 forms the central core of the plug 2, and includes pluralapertures for engaging assembly pins (not shown), which fix the variouselements together. All of the other elements (6, 8, 10, 14, 16, 18, 20,22, and 26) comprise an aperture sized to slide onto the mandrel 4.These elements generally slide together in a surface to surface manner.At the bottom of the mandrel 4 is a lower slip assembly 23, whichincludes a “mule shoe” bottom slip stop 26, which serves at the end ofthe plug assembly 2, and which is first inserted into the tubular as theplug 2 is lowered down hole. The lower slip assembly 23 further includesplural slips 24, which are inserted between the bottom slip stop 26 andthe bottom slip cone 22 that is also a part of the lower slip assembly23. This arrangement will be more fully discussed hereinafter.

The next group of components slid onto the mandrel 4 is the sealassembly 17, which includes the sealing member 18 and a pair of sealshoes 16, 20. The seal member 18 is a synthetic rubber that is suitablefor sealably engaging the interior circumference of a steel tubular. Thepair of seal shoes 16, 20 are conical members that advantageously engagethe seal member to affect the compression thereof. Above the top sealshoe 16 is the upper slip assembly 11, which comprises the upper slipcone, plural slips 12, and an upper slip stop 10. Additional stops 6, 8are employed in this embodiment as spacers to facilitate common partsusage between different size plugs. A single upper slip stop could alsohave been employed.

Reference is directed to FIG. 2, which is a partial exploded view of abridge plug according to an illustrative embodiment of the presentinvention. This view provides further details of the lower slip assembly24 from FIG. 1. In FIG. 2, the seal assembly 17 is also presented. Notethe configuration of the lower slip cone 22 and its relationship withthe plural slips 24, which comprises individual slip elements 28. Whenthe slip assembly 23 is assembled together, the slips 28 are supportedbetween the lower slip stop 26 and the lower slip cone 22. As the plugis set, the lower slip stop 26 is driven toward the lower slip cone 22,and this causes the outward radial movement of the slips 28 into thetubular (not shown). It is necessary to retain the slips 28 in place atthe time of manufacture, and through deployment into a down holetubular. In prior designs, one or more grooves are cut into the arcuateexterior face of each slip, which engaged one or more piano-wireswrapped about the circumference of the plug to hold the slips in place.In the present illustrative embodiment, the slips 28 are provided withcertain holes into which location pins are inserted, and this will bemore fully described hereinafter. In addition, polyolefin bands may beplaced over slips on the assembled plug, which are then heat-shrunken,to present a safe and smoothed exterior surface of the plug assembly.

Reference is directed to FIGS. 3A, 3B, 3C, and 3D, which are a frontview, side view, top view, and bottom view drawing, respectively, of abridge plug slip 28 according to an illustrative embodiment of thepresent invention. The configuration of the slip 28 is generallywedge-shaped, with an arcuate exterior face 38, which corresponds to theinterior circumference of the size tubular that the plug is designed tofit. The exterior face 38 also comprises plural teeth 30, which areangled in a sawtooth fashion to engaged the interior surface of atubular (not shown) in a directional manner, similar to the function ofa ratchet. The interior surface 40 of the slip 28 is also formed as anarcuate surface, which is configured to engage the conical shape of theslip cone (not shown). A pin hole 34 is formed through the slip 28,which serves as a location for insertion of a pin (not shown) betweenthe slip 28 and the slip cone (not shown). In addition, a pair of holes36 are formed into the top of the slip 28 for receiving a pair of pins(not shown) to engage the slip stop (not shown). The arrangement ofholes 34, 36 and pins (not shown) enable the attachment of the slips 28to the slip assembly. A groove 32 is formed vertically along theinterior arcuate surface 40 of the slip. The groove 32 engages a ridgeon the slip cone (not shown), and serves to both locate the slips 28with respect to the slip cone (not shown) and also to guide the movementof the slips 28 during the plug setting operation. This arrangement willbe more fully described hereinafter.

The material of construction and finishing techniques of the slips inFIG. 3A-D are significant features of the illustrative embodiment. Theslip 28 body needs adequate strength to endure the forces applied duringthe setting operation and the pressure holding operation, yet it isdesirable that the slip 28 be readily drillable. In addition, the teeth30 must have sufficient hardness to bite into the steel tubular. Thesegoals are advantageously achieved through use of sintered metal powderas the material of construction of the slips 28. Sintered metal iscomprised of powered metal that is fused into a solid shape through asintering operation, which yields an object with porous consistency. Theporosity results in a material that is more readily drillable ascompared to a solid objet made from the same material. There is also asignificant cost savings in that the complex shape of the slip 28 isformed through molding operations as opposed to machining operations.Although, some minor finishing operations may still be required with thesintered part. In the illustrative embodiment, the slips 28 are pressuremolded to produce a ‘green’ casting, which is then heat sintered topermanently fuse the powdered metal. In addition, a case hardeningtechnique may be applied to the exterior surface 38, particularlyincluding the plural teeth 30, to ensure that the hardness is sufficientto bite into the steel tubular surface.

In an illustrative embodiment, the metal powder used to mold the slips28 is primarily iron power, but with the addition of copper and carbon.The copper serves to enable the heat sintering operation at temperaturesjust over the melting point of copper. The carbon improves hardness ofthe sintered part, and particularly enables a heat-treating operation tocase harden the exterior face 38 and teeth 30 of the slips 28. The slips28 should have a surface hardness of at least fifty-five on theRockwell-C hardness scale, with the range of RHC 55 to 60 being asuitable engineering specification. Oxy-gas flame hardening with a rapidquench is a suitable approach to surface hardening, however, other casehardening techniques may be applied as well.

In an illustrative embodiment, the specification for the sintered metalis selected from the Metal Powder Industries Federation Standards,particularly MPIF Standard 35 for PM Structural Parts (ISBN No.978-0-9853397-1-5 (2012)). The Material Designation is FC-0208-65HT,which is a powdered metal comprised of 1.5 to 3.9% copper, 0.6 to 0.9%carbon, with the balance being powdered iron. The parts are pressuremolded and heat sintered, per the MPIF guidelines. Note that the minimumcarbon content specification of 0.6% places the material into the alloyrange for high carbon steel, which is particularly suitable for the casehardening operation applied to the teeth 30 of the slips 28. Withadequate hardening techniques, the case may be converted to martensite,a hardened steel, yet the interior of the slips remains a poroussintered material that is more readily drillable, as discussedhereinbefore. Oxy-gas flame or induction hardening can be employed.

Reference is directed to FIGS. 4A and 4B, which are a top view andsection view drawing, respectively, of a bridge plug slip cone 14, 22according to an illustrative embodiment of the present invention. Theslip cone 14, 22 illustrated in this drawing figure is suitable to serveas both the upper 14 and lower 22 slip cone, although, the two slipcones may differ to accommodate differences between the upper and lowerslip assembly positions. The slip cone 14, 22 body 40 has a aperture 44formed there through to accommodate the support mandrel (not shown).There is an upper conical portion 42, which serves as an inclined planeto drive the slips (not shown) into the tubular (not shown) when theplug is set. Plural ridges 46 are formed along the conical portion 42,which engage the slips (not shown) and serves to located and guide theslips during the setting operation. The number of ridges 46 correspondsto the number slips in the slip assembly, which in this embodiment iseight, although different numbers of slips may certainly be employed.Plural holes 48 are formed into the conical portion 42 and serve toengage locator pins (not show) between slip cones 14, 22 and the slips(not shown). In this embodiment, the holes 48 are aligned with the ridge46, however, this correspondence is not required. The aforementionedlocator pins (not shown) could be positioned elsewhere on the slip cones14, 22.

The choice of materials used for the slip cones 14, 22 in FIG. 4A-B is adesign choice. As was discussed hereinbefore, polymeric materials, whichmay be fiber reinforced are suitable choices. As mentioned hereinbefore,these include epoxy resins, phenolic resins, and other suitable organicplastics. Reinforcing fibers may be glass, carbon, aramid, or othersuitable non-metallic fibers. In an illustrative embodiment, the slipcones 14, 22 are fabricated from e-glass reinforced phenolic resin thatis cavity molded. The -glass fibers range in length from 0.5 to 10inches. Carbon fiber roving is another option. Glass spheres may beadded to the phenolic resin. E-glass is alumino-borosilicate glass withless than 1% alkali oxides, and is a common glass used in fiberreinforced polymers. However, other fiber reinforced plastic (FRP), suchas epoxy, vinyl ester, polyester thermosetting plastic, or phenolformaldehyde may be used. Hereinafter, the material used for the fiberreinforced organic plastics will be referred to as “FRP”, which meansany suitable fiber reinforced plastic, or suitable substitute, asdescribed herein.

Reference is directed to FIGS. 5A and 5B, which are a top view andsection view drawing, respectively, of a bridge plug slip stop 50according to an illustrative embodiment of the present invention. Inpractical implementations of the slip assembly according to theillustrative embodiments, the slip stops may take many forms toaccommodate other elements and functions of the plug. For example, thebottom slip stop 26 in FIGS. 1 and 2 serves as mule shoe that guides theassembly down hole. The embodiment of FIGS. 5A-B is an exemplar of ageneric slips stop 50. The slip stop 50 comprises an FRP body 52 thatpresents a donut configuration with a center aperture 54 to accommodatedthe plug mandrel (not shown). The slip stop includes plural holes 56formed therein to accept locator pins (not shown), which connect theslip stop 50 to the plural slips (not shown). Further explanation of theslip stop arrangements will be more fully developed hereinafter.

Reference is directed to FIG. 6, which is a side section view drawing ofa slip assembly in a well casing 60 according to an illustrativeembodiment of the present invention. The slip assembly comprises theslip stop 50, the slip cone 14, and plural slips 28. The slip assemblyis located on a plug mandrel 4, which is drawn in phantom to aid inclarity. FIG. 6 illustrates the slip assembly as it is provided from themanufacturer and before it is set into the tubular 60. In this drawing,only two opposing slips 28 are presented to aid in clarity of thedrawing. Note that the slip cone 14 has plural ridges 46 along itsconical portion. Also note that the grooves 32 on the interior of eachslip 28 engages the ridge 46, which thereby locates and guides themovement of the slips 28 along the slip cone 14 when the assembly is set(not shown in this figure). The slips 28 are fixed to the slip cone 14using one pin 64 per slip 28. The pins 64 are fabricated from FRP andare fixed with 5000 psi epoxy into the holes 48 in the slip cone and theholes 34 in the slips 28. This arrangement fixedly attaches the slips 28to the slip cone 14. In one embodiment, the pins 64 are one quarter inchin diameter.

The slips 28 in FIG. 6 are fixed to the slip stop 50 in a similar manneras they are attached to the slip cone 14. The slips 28 are fixed to theslip stop 50 using two pins 62 per slip 28. The pins 62 are fabricatedfrom FRP and are fixed with 5000 psi epoxy into the holes 56 in the slipstop 50 and the holes 36 in the slips 28. This arrangement fixedlyattaches the slips 28 to the slip stop 50. In one embodiment, the pins62 are one-eighth inch in diameter. All of the pins 62, 64 are selectedto have a shear strength that readily fails when the slip assembly isset into the tubular 60. Compressive forces from the setting tool (notshown) drives the slip stop 50 and the slip cone 14 toward one another.This action causes the slips 28 to ride up the conical portion of theslip cone 14 along the ridge 46 and groove 32, which action induces theshear forces on the pins 62, 64.

Reference is directed to FIG. 7, which is a top section view drawing ofa slip assembly in a well casing 60 according to an illustrativeembodiment of the present invention. FIG. 7 corresponds with FIG. 6 inthat FIG. 7 illustrates two of the slips 28 in solid line, and the othersix slips 29 are illustrated in phantom. The slip cone 14 is presented,including its central aperture 44 for engaging the mandrel (not shown).The plural ridges 46 on the slip cone 14 are visible, and well as theplural pin holes 48 along the conical portion 42 of the slip cone 14.With respect to the two slips 28 that are presented in solid line, theplural pin holes 56 can be seen as well as the end of the interiorgroove 32 on the slips 28. Also note the separation between the arcuateexterior surface 38 of the slips 28, and the interior circumference ofthe tubular 60.

Reference is directed to FIG. 8, which is a side section view drawing ofa deployed slip assembly in a well casing 60 according to anillustrative embodiment of the present invention. FIG. 8 correspondswith FIG. 6, where FIG. 8 shows the slip assembly after it has been setinto the tubular 60. Note the force arrows 65, which are presented toshow the direction the setting tool (not shown) forces act upon the slipcone 14 and slip stop 50. This action drives the two together, whichcauses the slips 28 to ride the slip cone 14 outwardly and engage theinterior circumference of the tubular 60. When the setting operationoccurs, the slip cone pins 64 shear, as do the slip stop pins 62. Thisaction separates the components of the slip assembly, which isbeneficial for the drilling operation. The slips 28 bite into thetubular 60 is a ratcheting fashion, such that the opposing slipassemblies (only one shown in FIG. 8), bindingly retain pressure on theseal assembly (not shown). Thusly, the slips 28 remain engaged with thetubular 60, and the seal assembly (not shown) remains compressiblysealed against the tubular 60 as well. This condition is maintaineduntil the plug is drilled out of the tubular, or is otherwise removed.Note that a suitable setting tool for a 5.5 inch well casing would drivethe forces 65 at 3500 psi. A four-inch stroke length setting tool wouldbe sufficient for the illustrative embodiment plug assembly. The typicalsetting distance for a two-slip assembly plug ranges from 1.5 to 3.0inches. The plug of the illustrative embodiment has been tested toexceed 25,000 psi differential pressure.

Reference is directed to FIG. 9 and FIG. 10, which are a top sectionview drawings of a slip stop assembly in a well casing 60 according toan illustrative embodiment of the present invention. FIG. 9 shows theslips 28 prior to being set in the tubular 60, and FIG. 10 shows theslips 28 after being set into the tubular 60. The slip cone 14 and itsconical portion 42 and central aperture 44 are also presented in thesedrawings. Note that the setting operation increases the spacing betweenadjacent slips 28.

Reference is directed to FIG. 11, which is a top section view drawing ofa deployed slip stop assembly in a well casing 60 according to anillustrative embodiment of the present invention. The illustrativeembodiment slip assembly comprises eight slips 28. Note that the slipsare not tightly spaces, as for example slices of pie, but rather aresized to provide spacing there between. This is a beneficial feature ofthe illustrative embodiment. Note in FIG. 11, that two of the slips havebeen rotated 68, 70. The clearance provided between adjacent slipsenables such rotation. And, such rotation allows a drill bit to moreeasily and freely grind the plug assembly into small enough debris toeither be pump out or to fall to the bottom of the tubular. It has beendetermined that a spacing which provides greater than 29% spacing alongthe interior circumference of the tubular 60 is sufficient to enablefree rotation of the slips 28 during the drilling operation. A suitabledesign criteria is to size the slips and spaces such that the sum lengthof the spaces between the plural slip elements 28 along the interiorcircumference of the tubular 60 accounts for thirty percent, and up tofifty percent, of the interior circumference length of the tubular 60.With this spacing arrangement, the drill bit will be considerably moreefficient at drilling out a set plug assembly.

Reference is directed to FIG. 12, which is a top section view drawing ofa deployed slip assembly in a well casing 70 according to anillustrative embodiment of the present invention. This embodimentillustrates an example where ten slips 76 are employed. The drawingillustrates three of the slips 78 as having been rotated by the drillingoperation. The drawing also illustrates a slip cone 72 with a conicalportion 72, and a central aperture 74. The number of slips, width, andspacing is a design choice, and can range from three to many.

Thus, the present invention has been described herein with reference toa particular embodiment for a particular application. Those havingordinary skill in the art and access to the present teachings willrecognize additional modifications, applications and embodiments withinthe scope thereof.

It is therefore intended by the appended claims to cover any and allsuch applications, modifications and embodiments within the scope of thepresent invention.

What is claimed is:
 1. A slip assembly for anchoring a sealing devicethat is assembled about a mandrel and set in place in a tubular with asetting tool, the tubular having an interior circumference, the slipassembly comprising: a slip cone fabricated from a polymeric materialand disposed along said mandrel; a slip stop fabricated from a polymericmaterial and disposed along said mandrel; plural slip elements, eachconfigured in a wedged shape, and having plural teeth disposed upon anarcuate surface thereof for selectively engaging the interiorcircumference of the tubular, and wherein said plural slip elements arepressure molded of powered iron that is sintered, and wherein saidplural slip elements are disposed between said slip cone and said slipstop, and spaced apart from each other, and wherein said teeth of saidplural slip elements are driven against the interior circumference ofthe tubular when said slip cone and said slip stop are driven toward oneanother along said mandrel by the setting tool, and wherein the sumlength of the spaces between said plural slip elements along theinterior circumference of the tubular accounts for greater than thirtypercent of the interior circumference length of the tubular.
 2. The slipassembly of claim 1, and wherein: said powered iron further containscarbon, and wherein said arcuate surface of said plural slip elements issurface hardened.
 3. The slip assembly of claim 2, and wherein: saidplural slip elements are molded from powdered iron comprising carbon inthe range of 0.6 to 0.9 percent by weight, and also comprising poweredcopper in the range from 1.5 to 3.9 percent by weight, and wherein saidpressure molded plural slip elements are heat sintered to a temperatureabove the melting point of copper.
 4. The slip assembly of claim 2, andwherein: said arcuate surface of said plural slip elements are surfacehardened by oxy-gas flame and rapidly cooled to yield a case surfacehaving a Rockwell C-scale hardness value greater than fifty-five.
 5. Theslip assembly of claim 1, and wherein: said sum length of the spacesbetween said plural slip elements along the interior circumference ofthe tubular accounts for greater than fifty percent of the interiorcircumference length of the tubular.
 6. The slip assembly of claim 1,further comprising: plural pins disposed between said plural slipelements and said slip cone to rigidly fix said plural slip elements tosaid slip cone, and wherein said plural pins have a shear strengthselected to shear under force of the setting tool.
 7. The slip assemblyof claim 1, further comprising: plural pins disposed between said pluralslip elements and said slip stop to rigidly fix said plural slipelements to said slip stop, and wherein said plural pins have a shearstrength selected to shear under force of the setting tool.
 8. The slipassembly of claim 1, further comprising: plural pins disposed betweensaid plural slip elements and said slip stop and said slip cone torigidly fix said plural slip elements to said slip stop and said slipcone, and wherein said plural pins have a shear strength selected toshear under force of the setting tool.
 9. The slip assembly of claim 8,and wherein: said plural pins are fabricated from fiber reinforcedpolymeric material.
 10. The slip assembly of claim 1, and wherein: saidslip cone and slip stop polymeric materials are fiber reinforced, andwherein the mandrel is fabricated from a fiber reinforced polymericmaterial.
 11. A slip assembly for anchoring a sealing device that isassembled about a fiber reinforced polymeric mandrel and set in place ina tubular with a setting tool, the tubular having an interiorcircumference, the slip assembly comprising: a slip cone fabricated froma fiber reinforced polymeric material and disposed along the mandrel; aslip stop fabricated from a fiber reinforced polymeric material anddisposed along the mandrel; plural slip elements, each configured in awedged shape, and having plural teeth disposed upon an arcuate surfacethereof for selectively engaging the interior circumference of thetubular, and wherein said plural slip elements are pressure molded frompowdered iron comprising carbon in the range of 0.6 to 0.9 percent byweight, and also comprising powered copper in the range from 1.5 to 3.9percent by weight, and wherein said pressure molded plural slip elementsare heat sintered to a temperature above the melting point of copper,and wherein said arcuate surface of said plural slip elements aresurface hardened by oxy-gas flame and rapidly cooled to yield a casesurface having a Rockwell C-scale hardness value greater thanfifty-five; plural pins, fabricated from a fiber reinforce polymericmaterial, disposed between said plural slip elements and said slip stopand said slip cone to rigidly fix said plural slip elements between saidslip stop and said slip cone, and wherein said plural pins have a shearstrength selected to shear under force of the setting tool, and whereinsaid plural slip elements are spaced apart from each other such that thesum length of the spaces between said plural slip elements along theinterior circumference of the tubular accounts for greater than thirtypercent of the interior circumference length of the tubular, and whereinsaid teeth of said plural slip elements are driven against the interiorcircumference of the tubular when said slip cone and said slip stop aredriven toward one another along said mandrel by the setting tool.